1. Field of the Invention
The present invention relates, in general, to fluidized bed gasification systems and in particular, to a new and useful self-contained system for gasifying a residual waste such as black liquor.
2. Description of the Related Art
In the pulp and paper production industry, recovery processes are used to generate steam and to recover certain chemicals used in the pulping process.
A common method of chemical recovery used in the Kraft Paper Pulping Process starts with the concentrating of what is known as black liquor. Black liquor is that stream which results from the washing of the pulp after the wood has been digested by the cooking liquor. This stream is rich in valuable chemicals which are recovered to produce some of the basic cooking liquor employed in the digester. This liquor also contains organic material picked up in the digestion process of the prepared woods. In a chemical recovery furnace black liquor, concentrated to approximately 60-80 percent solids by weight, is sprayed or introduced into the bottom portion of the furnace where smelt is formed. The smelt which is deposited in the bottom part of the furnace can be hazardous. If leaks develop in the water tubes, the probability of an explosion from the contact of the water and smelt is quite high. The flowing smelt is drawn off and treated so as to reconstitute the cooking liquor for the digestion process.
Other approaches to gasifying black liquor include high temperature (&gt;Tsmelt) entrained bed gasifiers and low temperature fluidized bed gasifiers. Some gasifiers directly input heat into the bed material itself, while others provide heat indirectly. Typically low temperature gasifiers provide heat input to the fluidized bed through the use of heat transfer surfaces such as heat exchanger tubes located in the bed material.
An integral part of gasification of fuels containing sulfur, especially black liquor gasification, is heat recovery and sulfur removal from the fuel (product) gas.
Gasification of black liquor is carried out by heating the liquor solids with a sub-stoichiometric amount of oxygen. The gasification products are a product gas stream rich in hydrogen, carbon monoxide, carbon dioxide and hydrogen sulfide and a stream of sodium salts. The sodium salts consist primarily of sodium carbonate with minor amounts of sodium-sulfur and potassium compounds. Most of the sulfur in black liquor is released to the gas phase as hydrogen sulfide in a low temperature gasifier.
Gasifier product gas temperature after typical conditioning, with a waste heat boiler or other heat trap, is 300F.-400F. Typical product gas scrubbing temperatures range from 100F.-180F. Thus, considerable sensible heat still exists in the product gas. Further, the product gas stream can contain considerable amounts of water vapor. When cooling to the required scrubbing temperature, considerable amount of latent heat is released.
The hydrogen sulfide (H.sub.2 S) in the gasifier product gas must be removed to produce a clean fuel gas that is suitable to burn in an auxiliary boiler or a gas turbine. The H.sub.2 S can be absorbed in a sodium carbonate solution formed by dissolving the solid product from the gasifier. The product solution from the scrubbing process would be a "green liquor" similar to the conventional kraft recovery process green liquor that can be re-used in the pulping process.
It is believed that existing pulp mill chemical solutions can be used as H.sub.2 S absorbing solutions. Gases containing both H.sub.2 S and CO.sub.2 will react with sodium salt solutions. CO.sub.2 absorption is not desirable as sodium bicarbonate is formed which will increase causticizing chemical usage and lime kiln capacity. Reaction rate data is limited on H.sub.2 S/CO.sub.2 /Sodium salt solution scrubbing systems. In U.S. Pat. No. 4,431,617 issued Feb. 14, 1984 and U.S. Pat. No. 3,471,249 issued Oct. 7, 1969, limiting CO.sub.2 absorption is discussed. Preferential absorption of H.sub.2 S over CO.sub.2 is a function of gas velocity, scrubbing temperature, mass transfer device, solution retention time, and scrubbing solution chemistry.
Presently, there is a need for a system and method for gasifying residual waste liquor which produces a product gas with low to medium heating value, and provides a high conversion of carbon and which uses low external heat input. Also, for a device and a process for removing sensible and latent heat from the product gas and controlling the absorption process for selective H.sub.2 S absorption. This apparatus and method should operate at a temperature so as not to produce smelt. Preferably, the system would be self-contained and function as a stand-alone system to process a residual waste liquor and generate a product gas for boilers, kilns, diesel generators, or gas turbine. Additionally, it should be environmentally friendly.